U.S. application Ser. No. 924,508, filed July 14, 1978 by the present applicant, now U.S. Pat. No. 4,191,547 discloses photo-sensitive glass compositions which demonstrate alterable photo-anisotropic effects, i.e., the glasses display photo-dichroic and photo-birefringent effects. Moreover, it was observed that polarization of those two effects could be reversibly altered between two arbitrarily chosen directions without apparent fatigue. Stated another way, those phenomena could be reversibly altered seemingly indefinitely with no substantial change in results. A specific utility disclosed for those inventive glasses was as optical recording media. Thus, that application describes the use of such compositions as a photosensitive optical information storage medium. Hence, the photo-anisotropic behavior exhibited by the glass permitted the storage of optical information therein.
The basis of that invention was the discovery that up to 25% by weight silver could be introduced into a surface layer of about 1-500 microns thickness in certain hydrated glass bodies of closely-defined compositions within the Na.sub.2 O and/or K.sub.2 O-ZnO-Al.sub.2 O.sub.3 -SiO.sub.2 -Cl field by means of an aqueous solution ion exchange reaction. That all or essentially all of the silver included in the thin, ion-exchanged surface layer is present in the ionic state is evidenced from the transparent, substantially colorless appearance of the integral surface layer.
The method disclosed for producing such bodies comprised three general steps:
(1) An anhydrous glass body consisting essentially, in mole percent on the oxide basis, of about 70-82% SiO.sub.2, 10-17% Na.sub.2 O and/or K.sub.2 O, 5-15% ZnO, 0.5-5% Al.sub.2 O.sub.3, and 0.1-3% Cl is formed.
(2) That glass body is contacted with an aqueous solution containing Ag.sup.+ ions acidified with an acid to a pH less than about 2 at a temperature in excess of 200.degree. C. and at a pressure in excess of 225 psig for a period of time sufficient to hydrate a surface layer thereon containing up to about 10% by weight H.sub.2 O and to effect the replacement of Na.sup.+ and/or K.sup.+ ions with Ag.sup.+ ions in the hydrated glass, the proportion of Na.sup.+ and/or K.sup.+ ions in the hydrated glass being less with a corresponding increase in Ag.sup.+ and/or H.sup.+ (H.sub.3 O.sup.+) ions. The Ag.sup.+ ions react with Cl.sup.- ions in the hydrated glass to bring about the formation of Ag-AgCl-containing crystals which impart photosensitivity to the glass.
(3) The hydrated, photosensitive surface layer is subjected to ultraviolet radiation at an intensity and for a period sufficient to develop photo-anisotropic properties therein.
With respect to the use of those glasses as optical recording materials, that application explicitly teaches that the optimum optical density demanded for optical recording can be secured in an ion-exchanged layer having a thickness greater than about 5 microns, and that the minimum spot size diameters which can be recorded in the surface layer employing a Gaussian beam at about 633 nm is 1.6 microns.